docs: def is_critical_flow(P1, P2, k):
      r'''Determines if a flow of a fluid driven by pressure gradient
      P1 - P2 is critical, for a fluid with the given isentropic coefficient.
      This function calculates critical flow pressure, and checks if this is
      larger than P2. If so, the flow is critical and choked.
  
      Parameters
      ----------
      P1 : float
          Higher, source pressure [Pa]
      P2 : float
          Lower, downstream pressure [Pa]
      k : float
          Isentropic coefficient []
  
      Returns
      -------
      flowtype : bool
          True if the flow is choked; otherwise False
  
      Notes
      -----
      Assumes isentropic flow. Uses P_critical_flow function.
  
      Examples
      --------
      Examples 1-2 from API 520.
  
      >>> is_critical_flow(670E3, 532E3, 1.11)
      False
      >>> is_critical_flow(670E3, 101E3, 1.11)
      True
  
      References
      ----------
      .. [1] API. 2014. API 520 - Part 1 Sizing, Selection, and Installation of
         Pressure-relieving Devices, Part I - Sizing and Selection, 9E.
      '''
      Pcf = P_critical_flow(P1, k)
      return Pcf > P2